For example, in the case where an international call or the like is made, voice quality becomes a problem. As means for preventing the deterioration of the voice quality in such a long-distance call, there is a tandem pass through function. FIG. 3 is a diagram showing a transmission mode of voice signals by a general tandem pass through function.
As shown in FIG. 3, one side of a DCME (digital circuit multiplication equipment) 3a as a kind of voice transmission device is connected to a terminal 1a through an exchange 2a, and the other side thereof is connected to a DCME 3b. Each of the DCMEs 3a, 3b, 3c and 3d has the tandem pass through function. As expressed in a block diagram indicating a structure of a conventional DCME shown in FIG. 4, the respective DCMEs 3a, 3b, 3c and 3d include subscriber side I/F parts 11a, 11b, 11c and 11d, echo cancellers 12a, 12b, 12c and 12d, voiced/unvoiced detection parts 13a, 13b, 13c and 13d, voice coders 14a, 14b, 14c and 14d, line side I/F parts 15a, 15b, 15c and 15d, bit detection parts 16a, 16b, 16c and 16d, tandem pass through detection parts 17a, 17b, 17c and 17d, voice decoders 18a, 18b, 18c and 18d, signal synthesis parts 19a, 19b, 19c and 19d, and pattern bit generation parts 20a, 20b, 20c and 20d, respectively.
First, a case where the tandem pass through function is not used, for example, a case where a voice call is made from a terminal 1a to a terminal 1d will be described. A voice call signal of the terminal 1a is inputted to the DCME 3a through an exchange 2a, and is coded in the coding part 30a. When inputted to the DCME 3b at the other end by wire, it is decoded in the decoding part 40b and is returned to original voice data, and voice is reproduced through an exchange 2b by the terminal 1d at the other end.
On the contrary, a voice call signal from the terminal 1d at the other end is inputted to the DCME 3b through the exchange 2a, is coded in the coding part 30b, is inputted to the DCME 3a by wire, is decoded in the decoding part 40a and is returned to the original voice data, and voice is reproduced through the exchange 2b by the terminal 1a. 
In general, two such DCMEs form a pair and perform a call operation with the terminal at the other end. However, for example, like a case where with respect to the terminal 1a, a terminal at the other end becomes a terminal 1f, in the case where a call operation in a multistage connection state (in this case, two-link connection) is performed by an international call or the like, it is necessary to use the exchanges 2a, 2b, 2c and 2d and the DCMEs 3a, 3b, 3c and 3d. At that time, since decoding and coding are performed at each stage of the DCMEs 3b and 3c, the voice quality is deteriorated by that.
Thus, when a voice call signal transmitted from the terminal 1a to the terminal 1f is not decoded in the decoding part 40b of the DCME 3b, and is not coded in the coding part 30c of the DCME 3c, the voice call signal is merely coded in the coding part 30a of the DCME 3a, and is decoded in the decoding part 40d of the DCME 3d, and therefore, the deterioration of the voice quality can be suppressed.
Then, a case where a voice call is performed between the terminal 1a and the terminal 1f will be described. First, the voice call signal from the terminal 1a is inputted to the DCME 3a through the exchange 2a. In general, plural 2.048 MHz (E1) signal lines or 1.544 MHz (T1) signal lines based on ITU-T recommendation G.703/G.704 are inputted from the exchange side to the subscriber side I/F part 11a for capturing the signal from the subscriber side exchange 2a. In the signal lines, in general, one line is multiplexed into 30 channels of voice signals in the case of E1, and one line is multiplexed into 24 channels in the case of T1. The subscriber side I/F part 11 decomposes the inputted voice signal for each channel, and inputs it to the coding part 30a. 
In the coding part 30a, the voice call signal is first inputted to the echo canceller 12a and the bit detection part 16a. In the echo canceller 12a, an echo cancel processing is performed, and the signal after the processing is inputted to the voiced/unvoiced detection part 13a and the voice coder 14a. The voiced/unvoiced detection part 13a judges a voiced part and an unvoiced part of the voice call signal, and outputs the judgment result to the voice coder 14a. The voice coder 14a performs coding of only the voiced part from the inputted voice call signal and the voiced and unvoiced judgment result.
On the other hand, in the bit detection part 16a, as shown in FIG. 5, predetermined two bits in the 8-bit basic cell of the PCM signal as the voice call signal are extracted, and the extracted two bits are outputted to the tandem pass through detection part 17a. In the tandem pass through detection part 17a, the extracted 2-bit signal is stored, and it is detected whether in a state of time-series arrangement, there is an establishment pattern of a predetermined tandem pass through state. However, in this case, in the DCME 3a, since the voice call signal from the terminal 1a is merely sent, the establishment pattern is not detected. Accordingly, the echo canceller 12a and the voice coder 14a continue to operate as long as a call is performed.
From the above, the voice call signal coded by the voice coder 14a is outputted to the decoding part 40b of the DCME 3b from the line side I/F part 15a by wire.
The coded voice call signal inputted to the decoding part 40b of the DCME 3b is inputted to the voice decoder 18b in the decoding part 40b until the tandem pass through state is established, and decoding of the voice is performed. The decoded voice is inputted to the signal synthesis part 19b, is synthesized every two bits with the establishment bit pattern indicating the establishment of the tandem pass through state, which is generated in the pattern bit generation part 20b in the decoding part 40b and is to be transmitted to the DCME 3c, and is outputted from the subscriber side I/F part 11b to the coding part 30c of the DCME 3c by wire. The establishment bit pattern is a specific pattern constructed of a predetermined number of bits, and is sequentially embedded every two bits in the 8-bit basic cell of the PCM signal as the voice call signal in the signal synthesis part 19b. 
On the other hand, in the case where the tandem pass through state is established between the DCME 3b and the DCME 3c, the operation of the voice decoder 18b is stopped by the control signal from the tandem pass through detection part 17b. An operation at the time when this tandem pass through state is established will be described.
As described above, at the time of start of a call, that is, in the case where the tandem pass through state is not established, the voice call signal including the establishment bits is outputted from the decoding part 40b of the DCME 3b to the coding part 30c of the DCME 3c, and on the other hand, the voice call signal including the establishment bits is outputted also from the decoding part 40c of the DCME 3c to the coding part 30c of the DCME 3b. 
When detecting the reception of the establishment bit pattern from the DCME 3c by the extraction of the bit detection part 16b of the DCME 3b, the tandem pass through detection part 17b immediately outputs a control signal to the pattern bit generation part 20b in the decoding part 40b of the DCME 3b, and the pattern bit generation part 20b outputs a detected bit pattern different from the establishment bit pattern in pattern. The DCME 3c also receives the establishment bit pattern from the DCME 3b, so that the detected bit pattern is outputted from the decoding part 40c to the coding part 30b of the DCME 3b. 
In this way, the detected bit pattern is outputted from the coding part 30b of the DCME 3b to the decoding part 40c of the DCME 3c, and the detected bit pattern from the coding part 30c of the DCME 3c is inputted to the decoding part 40b of the DCME 3b. As stated above, when there occurs a state in which both the coding part 30b and the decoding part 40b output the detected bit patterns and receive them, the tandem pass through detection part 17b judges that the tandem pass through state is established, outputs the control signal to the echo canceller 12b, the voice coder 14b, and the voice decoder 18b, stops the operations of these, and performs a control so that the inputted signal is outputted as it is. After that, until the time of the end of the call, as long as the state continues in which both the coding part 30b and the decoding part 40b output the detected bit patterns and receive them, the operations of the echo canceller 12b, the voice coder 14b, and the voice decoder 18b are stopped, and the inputted signal is outputted as it is.
In the coding part 30c of the DCME 3c, at the time of the start of the call, that is, when the tandem pass through state is not established, the voice call signal in which the establishment bit pattern is inserted is inputted to the echo canceller 12c and the bit detection part 16c. 
The bit detection part 16c extracts the predetermined two bits and outputs them to the tandem pass through detection part 17c. The tandem pass through detection part 17c stores the extracted bits, and detects the establishment bit pattern transmitted from the DCME 3b, and when detecting, it immediately controls the pattern bit generation part 20c in the decoding part 40c, and changes the output of the establishment bit pattern to the output of the detected bit pattern. In this way, similarly to the above, when the state occurs in which the decoding part 40c of the DCME 3c outputs the detected bit pattern to the coding part 30b of the DCME 3b, and the detected bit pattern is inputted in the coding part 30c of the DCME 3c from the decoding part 40b of the DCME 3b, the tandem pass through detection part 17c first stops the operation of the echo canceller 12c, the voice coder 14c and the voice decoder 18c, and outputs the control signal so that the inputted signal is outputted as it is.
In this way, before the establishment of the tandem pass through state, the coding part 30c of the DCME 3c codes only the voiced part of the voice call signal, and outputs it to the decoding part 40d of the DCME 3d, and after the establishment of the tandem pass through state, the coding part outputs the inputted signal to the decoding part 40d of the DCME 3d as it is.
In the decoding part 40d of the DCME 3d, the coded voice call signal is inputted to the voice decoder 18d through the line side I/F part 15d, is decoded by the voice decoder 18d and is outputted to the signal synthesis part 19d. In the signal synthesis part 19d, the voice call signal is synthesized with the establishment bit pattern indicating the establishment of the tandem pass through state and generated in the pattern bit generation part 20d in the decoding part 40d, and is outputted to the terminal 1f through the exchange 2d from the subscriber side I/F part 11d. The establishment bits synthesized here and outputted to the terminal 1f are neglected at the time of voice reproduction. As stated above, the decoding part of each of the DCMEs outputs the establishment bit pattern irrespective of whether the party at the other end is a terminal or another DCME.
In the above, although the operation concerning the outward path from the terminal 1a to the terminal 1f has been described, the return path will next be described.
The voice call signal from the terminal if is outputted through the exchange 2d to the coding part 30d of the DCME 3d, and similarly to the case of the DCME 3a of the outward path, its echo component is removed by the echo canceller 12d, only its voiced part is subjected to the coding processing by the voiced/unvoiced detection part 13d and the voice coding part 14d, and it is outputted to the decoding part 40c of the next stage DCME 3c. 
The decoding part 40c establishes the tandem pass through state as described above. Accordingly, just after the call start and before the establishment of the tandem pass through state, the voice is decoded by the voice decoder 18c, the pattern bit generation part 20c outputs the establishment bit pattern, the establishment bit pattern is inserted in the voice call signal in the signal synthesis part 19c, and it is outputted to the coding part 30b of the DCME 3b. On the other hand, after the establishment of the tandem pass through state, by the control of the tandem pass through state detection part 17c, the voice decoder 18c does not perform the decoding processing, and is switched to such an operation that the inputted signal is outputted as it is. The pattern bit generation part 20c outputs the detected bit pattern to the signal synthesis part 19c. In the signal synthesis part 19c, the signal not subjected to the decoding processing and the detected bit pattern are synthesized, and are outputted to the coding part 30b of the DCME 3b. 
The establishment of the tandem pass through state of the DCME 3b is as described above. In the coding part 30b of the DCME 3b, similarly to the case of the DCME 3c of the outward path, before the establishment of the tandem pass through state, only the voiced part of the voice call signal is coded and the signal is outputted to the decoding part 40a of the DCME 3a, and after the establishment of the tandem pass through state, the inputted signal is merely outputted to the decoding part 40a of the DCME 3a as it is.
In the decoding part 40a of the DCME 3a to which the outputted voice call signal is inputted, similarly to the above, the inputted voice call signal is decoded and is outputted to the terminal 1a. Since only the voice call signal from the terminal 1a is inputted to the coding part 30a of the DCME 3a, the tandem pass through state is not established as stated above, and accordingly, the decoding operation is not stopped in the decoding part 40a of the DCME 3a. 
As described above, with respect to the establishment of the tandem pass through state, the establishment bit patterns are outputted from the respective decoding parts, the detected bit pattern is returned in the case where the other DCME receives the establishment bit pattern, and the tandem pass through state is made to have been established during the period when the detected bit patterns are mutually outputted. Accordingly, immediately after the call start, coded voice is transmitted plural times, however, since the tandem pass through state is immediately established, there is substantially no influence at the time of the call.
By doing so, eventually, even in the case where the DCMEs for two links are connected to one another, the voice data from the terminal 1a to the terminal 1f is subjected to the voice coding and decoding only once in the DCMEs 3a and 3d, and accordingly, the deterioration of the voice quality can be prevented.
The conventional DCME with the tandem pass through function as stated above performs such a processing that the voiced part of the voice signal is detected by using the voiced/unvoiced detection part 13 of FIG. 4, and only the voiced portion is compressed and coded, and is transmitted to the line side. Thus, in the case where the voiced ratio (ratio of the voiced time to all call time) of the voice signal from the subscriber side is high, the transmission amount to the line side is eventually increased.
In general, in a call made using an indoor fixed phone, the voiced ratio of the voice is about 40%, and on the other hand, in a call made using a cellular phone remarkable popularized in recent years, the call environment is often the open air, and in that case, the background noise (for example, sound from a passing automobile, conversations among persons other than the caller) is large. As a result, it is said that the voiced ratio of the voice is about 66%, the transmission amount to the line side is increased, and the transmission efficiency of the line is lowered.
The invention has an object to improve the transmission efficiency of a line by removing a background noise component of a voice signal in a voice transmission device having a tandem pass through function.